In general, electric motors operate by rotating a rotor relative to a fixed stator by varying the orientation of a magnetic field induced by one or more coils. In some electric motors, both the rotor and stator include coils. In such an induction motor, the magnetic field induced by the stator coils induces current within the rotor coils which, due to Lenz's law, causes a resultant torque on the rotor, thus causing rotation.
In a permanent magnet motor, on the other hand, the rotor includes one or more permanent magnets. The permanent magnets, in attempting to align with the magnetic field induced by the coils in the stator, cause a resultant torque on the rotor. By varying the orientation of the magnetic field, the rotor may thus be caused to rotate. In high-torque permanent magnet motors, multiple permanent magnets may be positioned on the exterior of the rotor (for an internal rotor permanent magnet motor).
While in operation, the components of the permanent magnet motor may heat up in response to, for example, electrical resistance in the stator coils, losses in iron core of stator, induced currents in rotor caused by harmonics, mechanical friction, etc. Because of this increase in heat, the permanent magnets must be bonded to the rotor in such a way that any thermal expansion of the rotor or permanent magnets will not cause the permanent magnets to fracture or separate from the rotor. Additionally, in cases where the permanent magnets are formed by, for example, sintering, the permanent magnets themselves may be relatively brittle. Furthermore, where the permanent magnets are constructed of a material with a different thermal expansion coefficient than the rotor, as is often the case, the thermal expansion of the rotor may cause the permanent magnets to crack.